The present invention relates to testing apparatus, and more particularly to fatigue testing apparatus in which a specimen is subjected to both a static and dynamic load for crack initiation and crack propagation studies.
Many types of machinery include components which must withstand cyclical stressing at high speeds. For example, an aircraft jet engine includes rotating components such as turbine blades and discs which rotate at thousands of revolutions per minute. Such components are subjected to low cycle, mostly static stresses due to centrifugal forces and high cycle, mostly dynamic stresses due to resonance responses at high frequencies, as well as thermal excursions. These thermo-mechanical loads can initiate or propagate cracks at preexisting defects or scratches, and thereby reduce the fatigue life of the components.
In actual use, typical aircraft engine blade or disc materials may be subjected to static and dynamic stresses of 100,000 psi and 20,000 psi, respectively, due to centrifugal and resonance effects. The dynamic stresses might occur at any potential resonance frequency, from 100 Hz for long fan blades to 5,000 Hz for short turbine blades to 10,000 Hz for ultrashort engine blades of the type used on the space shuttle. Accordingly, there is a need for test systems which can impose both low frequency and high frequency loads on a specimen simultaneously.
Various devices have been developed to impose both static and dynamic loads upon a specimen. For example, in one device a specimen is attached at one end to a support frame and at its other end to a high capacity electrodynamic snaker which generates a high cycle load. The shaker is attached to a servohydraulic system which develops a low cycle load, and an intermediate rubber isolator is included in the connection between the shaker and servohydraulic system to prevent transmission of high cycle vibrations generated by the shaker back to the servohydraulic system.
In another device, a specimen is attached at one end to a support frame and at its other end to the rod of a piston within a cylinder pressurized by compressed air which exerts the low cycle load. A shaker is directly coupled to the piston and exerts the high cycle load.
A disadvantage with both of the aforementioned systems is that they have limited frequency capability and nonlinear calibration characteristics beyond approximately 1,000 Hz, and the latter system experiences a high cycle load capacity drop at frequencies higher than about 1,000 Hz. Furthermore, with an upper limit of about 1,000 Hz, it would take almost three hours to complete a ten million cycle test.
Another disadvantage with the aforementioned devices is that in both, the specimen is rigidly attached at one end to the support frame. Consequently, the imposition of high frequency stressing on the specimen causes the frame to vibrate along with the specimen and thereby affects the accuracy of the testing of the specimen. Furthermore, in crack propagation studies, the growth of the crack in the specimen affects the resonant frequency of the specimen and thereby changes the resonant characteristics of the entire system. This, too, affects the overall accuracy of the testing.
An additional disadvantage with the aforementioned devices is that, if the specimen tested is to be subjected to thermal loads by enclosing it in a furnace, a closed loop computer control is necessary to continuously adjust the position of the servohydraulic piston to maintain constant load on the specimen independently of the effects of thermal expansion. Consequently, the need for placing sensors on the apparatus within the furnace would greatly complicate the system.
Accordingly, there is a need for a fatigue testing apparatus in which both the specimen and the components exerting the high and low cycle loads are isolated from the support frame so that the support frame does not affect the dynamic characteristics of the load train. Furthermore, there is a need for a testing apparatus in which high cycle loads in excess of 1,000 Hz can be imposed upon a specimen, simultaneously with low cycle static loads, and yield accurate results which are substantially unaffected by changes in resonance frequency of a specimen caused by crack propagation.